b)Ta có:2^10=1024;5^5=3125
--> 2^10x3<5^5
-->(2^10x3)^172<5^5x5^172
2^1720x3^172<5^860
Lại có:3^7=2187
2^11=2048
-->3^7>2^11
3^172=(3^7)^24x3^4>(2^11)^24x3^4>2^264x2^6=2^270
2^1720x2^270<2^1720x3^172<5^860
-->2^1990<5^860
Mà 2^5<5^3(vì 32<125)
-->2^1990x2^5<5^860x5^3
hay 2^1995<5^863
Nếu pạn thấy đúng thì k cho mk nhé!!!!!!!